Modified atmosphere packaging, and in particular oxygen-free packaging, are growing in importance for extending the shelf-life of perishable products, such as foodstuffs, or providing a protective atmosphere for other types of sensitive products. However, this growth has been limited by the absence of suitable sensors that can confirm that the oxygen levels are kept at suitably low levels.
This failure in the art is a result of the diverse range of requirements for an ideal sensor. Generally, the sensors need to be inexpensive as packaging cannot incur high costs, especially for consumable products such as foodstuffs. They also need to be safe and non-toxic as they may be in contact with food and leaching may occur, or they may be at risk of accidental ingestion, for example by children. Furthermore the reaction of the sensor to a packaging failure preferably needs to be irreversible under normal packaging environment. This is important as an increase in oxygen level when a packaging failure occurs can lead to spoilage due to growth of microorganisms and it can happen that their metabolic activity restores low oxygen levels. This means that a reversible sensor may show that no packaging failure has occurred. In addition, it would be useful if the sensitivity of the sensor was tailorable as different applications of the packaging may tolerate different oxygen levels. Finally, the sensors should ideally be easy to use, providing an observable change that an unsophisticated user can appreciate, and without recourse to additional equipment, and be easy to incorporate in the packaging.
There are a few products commercially available in the market, namely, Oxy2 Dot® RedEye® and Ageless Eye™. However, they suffer from a range of significant limitations.
The Oxy2 Dot®, which is manufactured by OxySense®, is a fluorescence-based oxygen sensor. The product is a small circle that sticks to the inside of the package and the fluorescence intensity, which is inversely proportional to oxygen level, is measured by a photodetector. The two main problems with this sensor are (a) the need for expensive sensing equipment, and (b) temporary high oxygen levels may go unnoticed as the sensor operates in a reversible manner. The RedEye® operates on similar principles to the Oxy2 Dot® and, therefore, suffers from similar limitations.
The Ageless Eye™, which is manufactured by Mitsubishi Gas Chemical (MGC), changes visible colour upon oxidation. At low oxygen concentrations (<0.1%) the sensor appears pink, but when the oxygen concentration increases the colour changes gradually to blue. The limitations of this sensor are that it is not irreversible, it is expensive (60 p each) and the dye is harmful (methylene blue).
A further product is under development. A UV activated oxygen indicator is currently being developed by Mills. This sensor, which uses the same dye as Ageless Eye™ (methylene blue), is coated onto the inner side of packaging film and is only activated when exposed to UV light, which changes its colour from blue to white. When the sensor is exposed to oxygen, it regains the blue colour. The main issues with this sensor are safety issues due to the use of methylene blue, aesthetic (tainting of food), and the bleaching effect (blue to white) due to prolonged exposure to shelf light, which may lead to false negatives.
There is also a need to develop an effective oxygen sensor for use in non-food related fields, such as packaging pharmaceuticals and nutraceuticals, and for use in other fields, such as the storage of rare books and manuscripts, and for use in the packaging of high value products, such as electronic devices and components.
Oxygen sensing has been developed in other technical fields. U.S. Pat. No. 3,663,176 describes the use of metal salts of elements in Group IVB and VB of the periodic table as a colorimetric oxygen indicator in stream of alkene (olefin) in polymerization processes. US 2008/0300133 discloses an oxygen scavenger and indicator comprising three components: (a) an oxygen sorbent which is a metal or metal compound that can transfer from one oxidation state to another, (b) a redox indicator and/or complexing agent for the metal or the oxidised form of metal, and (c) at least one polymer or gel electrolyte. The oxygen indicator of this application apparently works when the oxidation of the metal causes a change in a physical property of the oxygen sorbent through a change in the interaction with the redox indicator or the complexing agent, such as a colour change.
GB 2,369,808 discloses oxygen or water sensors for food packaging based on a colour change of soluble transition metal compound, generally a soluble coordination complex.
From the discussion above, it will be apparent that the provision of an effective sensor for modified atmosphere packaging, and in particular for packaging for foodstuffs, remains an unsolved problem in the art.